Thermally responsive electrical switches

ABSTRACT

A thermally-responsive current-sensitive switch having a molded plastic body containing first and second terminals in the form of metal stampings and having an internal chamber wherein a leaf spring and a snap-acting bimetallic blade of the same shape are together spot-welded to one of the terminals with the leaf spring underlying the bimetal in registry therewith and at its other end carrying a sliver contact which cooperates in switching operations with the other switch terminal. In one embodiment, the spring bias of the leaf spring tends to open the switch and the force generated by the bimetal in its contacts-closing movement is sufficient to overcome the spring bias of the leaf spring, whereas in another embodiment the leaf spring develops either a neutral force or a contacts-opening force. The leaf spring electrically shunts the bimetal, whereby the current-carrying capability of the switch is increased and different switch specifications can use the same bimetal with different leaf springs.

FIELD OF THE INVENTION

This invention concerns improvements relating to electric switches andmore particularly concerns thermally responsive electrical switchesemploying bimetallic elements as thermal actuators.

BACKGROUND OF THE INVENTION

Many kinds of electrical switches employing bimetallic actuators areknown and likewise many different forms of bimetallic switch actuatorsare known. Early bimetallic switches simply employed a plain bimetalblade which moved relatively slowly in response to temperature changesand gave rise to arcing problems in the switch, and the development ofthe snap-acting bimetallic actuator, constructed as a dished bimetallicelement capable of moving between oppositely curved configurations witha snap action, provided a major advance in the art. Various forms ofsnap-acting bimetallic actuators are known, such as those disclosed inGB 600055, GB 657434, GB 1064643, GB 1542252 and GB 2124429 for example.Likewise, various forms of electric switches employing such bimetallicactuators are known; GB 2124429 abovementioned for example discloses theutilization of a pear-shaped snap-acting bimetallic actuator in acurrent-sensitive switch where the heating of the bimetal by flow ofelectric current therethrough is designed to trip the switch in acurrent overload situation.

In WO 92/20086 there is described a miniature electrical switchemploying a snap-acting bimetallic actuator, the switch being wellsuited to automatic manufacture and installation and comprising aminimum number of parts. The switch comprises a moulded plastics bodyportion which captures therein first and second terminal conductors, anda snap-acting bimetallic actuator is secured to one of the twoconductors and carries a contact which constitutes the moving contact ofthe switch and is arranged for co-operation in switching operations withthe other of the two conductors. The switch can be made highly currentsensitive by use of a bimetallic material of very low thickness (eg.0.003 inch: 0.076 mm) and the internal construction of the switch bodycan be designed to provide physical support for such a thin bimetallicelement. The possibility is further described of providing a silver orsilver alloy coating, for example a silver antimony coating as describedin WO 92/14282 on the terminal conductor which co-operates with themoving switch contact carried by the bimetal so as to enable anotherwise plain conductor to be utilized without need for attachment ofa discrete contact to the conductor.

To enhance the current sensitivity of the switch, the possibility isfurther disclosed to provide a series-connected heating element in theswitch for injecting heat into the bimetallic actuator when the switchis in closed condition, and in a particularly convenient arrangementthis is achieved by forming the heating element as a portion of one orother, or both, of the two terminal conductors. Yet another possibilityis to provide a heating element in parallel with the switch conductors,for example by use of a conductive ink printed on the switch bodyportion, the effect of this being to inhibit resetting of the switch solong as its power supply remains connected.

OBJECTS AND SUMMARY OF THE INVENTION

One object of the present invention is to enable the provision of athermally-responsive switch incorporating a bimetallic switch actuator,the switch obtaining the advantages of the switch of WO 92/20086abovementioned and furthermore providing for increased current carryingcapacity and preferably, though not essentially, for increased firstbreak times to avoid nuisance tripping of the switch.

According to the present invention, in one of its aspects, there isprovided a thermally-responsive switch comprising a moulded plasticsbody portion capturing therein first and second terminal conductors, aleaf spring secured to one of said conductors and carrying a contactwhich constitutes the moving contact of the switch and is arranged forco-operation in switching operations with the other of the twoconductors, and a snap-acting bimetallic actuator secured to said one ofsaid conductors and co-operating with said leaf spring to determine thecondition of the switch, the bimetallic actuator being electricallyshunted by the leaf spring.

In a first embodiment of the invention which is described in detailhereinafter, the snap-acting bimetallic actuator of a switchsubstantially as described in WO 92/20086 is shunted by means of anelectrically conductive leaf spring which carries the moving contact ofthe switch, the leaf spring underlying the bimetallic actuator andgenerating a spring force which is directed so as to tend to open theswitch but will be overcome by the force developed by the bimetallicactuator in its cold condition.

More particularly, the aforesaid embodiment of the present inventioncomprises a moulded plastics body portion capturing therein first andsecond terminal conductors, a leaf spring secured to one of saidconductors and carrying a contact which constitutes the moving contactof the switch and is arranged for co-operation in switching operationswith the other of the two conductors, and a snap-acting bimetallicactuator secured to said one of said conductors and co-operating withsaid leaf spring to determine the condition of the switch, the leafspring generating a spring force tending to open the switch contacts andthe bimetallic actuator overlying the leaf spring and developing in itscold condition a force such as to overcome the spring force of the leafspring, the bimetal being electrically shunted by the leaf spring.

The operation of the leaf spring in this embodiment in opposition to thebimetal ensures that when operated the spring remains in contact withand electrically shunts the bimetal, thereby increasing the currentcarrying capacity of the switch and increasing first break times sinceonly a proportion of the through current of the switch flows in thebimetal. Furthermore, when the bimetal moves to its hot condition thespring force generated by the leaf spring will tend to oppose resettingof the bimetal, thereby extending the off time of the switch which isadvantageous in protective applications.

In an alternative embodiment of the present invention which is describedhereinafter, the leaf spring does not necessarily oppose the bimetalwith a switch-opening spring force, but rather may be neutral or mayeven develop a switch-closing spring force, and the leaf spring andbimetal are coupled together in a manner which accommodates theirindividual movements whilst ensuring that they move together inswitching operations. The leaf spring has an end portion which is turnedupwards out of the general plane of the spring and an aperture is formedin such end portion, and the bimetallic actuator has a portion whichextends through the aperture in the leaf spring end portion with asufficient clearance to ensure that there is substantially no resistanceto relative movement between the leaf spring and the bimetallic actuatorgenerally in the direction of their longitudinal extent. Alternativemeans of coupling the leaf spring and the bimetallic actuator will occurto those possessed of relevant skills; for example the leaf springand/or the bimetallic actuator could be provided with one or more tabsfolded over so as loosely to entrap the other part.

By arranging the leaf spring so that it develops a switch-closing springforce, in contrast to a neutral or switch opening force, the advantageis obtained that an increased contact pressure can be achieved in theswitch when the temperature is close to the break temperature at whichthe switch will be opened by the bimetallic actuator, particularly whena snap-acting bimetallic actuator is employed. By this means a preciseand predetermined switch action can be achieved which is not susceptibleto creepage problems.

The leaf spring and the bimetallic actuator in both of theaforementioned embodiments may advantageously be similarly shaped andmay even be produced by use of the same tooling in an automatedmanufacturing procedure employing interchangeable punches and/or dies.Thus in the embodiment of the invention which are described hereinafter,the bimetallic actuator comprises a dished bimetallic blade having agenerally U-shaped cut-out defining a central tongue extending between apair of external legs which are bridged by a bridging portion adjacentthe tip of the tongue, and the leaf spring is substantially identicallyshaped and lies beneath the bimetallic actuator in registry therewith.The tongue of the bimetallic blade and the corresponding part of theleaf spring are commonly secured to said one of said conductors, forexample by spot welding, and the moving contact of the switch is carriedby the part of the leaf spring that corresponds to the bridging portionof the bimetallic actuator. The moulded plastics body portion of theswitch accommodates the bimetallic actuator and the leaf spring, and anupstand is defined within said chamber and provides support for thetongue of the bimetallic blade and for the corresponding part of theleaf spring where they attach to the terminal conductor. With suchsimilar bimetallic blade and leaf spring configurations, the bimetal andthe leaf spring can advantageously be assembled together, as asub-assembly, prior to their assembly together into the switch, whichhas advantages as regards the assembly of the switch.

Similarly to the invention described in WO 92/20086, the mouldedplastics body portion of the switch preferably defines a closed chamberwhich accommodates the leaf spring and the bimetallic actuator, thefirst and second terminal conductors are moulded into the body portionof the switch at spaced-apart locations so as to have exposed portionsspaced apart from each other within said chamber and externally of thebody portion, the bimetallic actuator and the leaf spring are directlywelded to the exposed portion of said one of said conductors within thechamber, and the contact carried by the leaf spring co-operates with acontact defined by or provided on the exposed portion of the other ofthe two conductors within the chamber.

Further advantages arise by virtue of the fact that switches havingdifferent switching characteristics can be obtained according to theteachings of the present invention simply by use of different leafsprings having different electrical and/or physical properties,advantageously without need to change the bimetal though this wouldprovide additional possibilities, and in that the provision of the leafspring avoids the need for welding of the moving contact of the switchto the bimetal (bimetallic materials are generally difficult weldingmaterials).

The above and further features and aspects of the present invention areset forth with particularity in the appended claims and will be madeclear in the following description of exemplary embodiments of theinvention which are illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E show the moulded plastics body part of a first exemplaryswitch in accordance with the present invention, with a closure memberomitted and showing first and second terminal conductors captured in thebody part, and with the leaf spring and bimetallic actuator of theswitch also omitted, FIG. 1A being a top plan view, FIGS. 1B and 1Cbeing cross-sectional side elevation views on the lines A . . . A and B. . . B respectively in FIG. 1A, FIG. 1D being a cross-sectional endelevation view on the line C . . . C in FIG. 1A, and FIG. 1E being abottom plan view of the switch of FIG. 1A;

FIG. 2 is an enlarged showing of the shape of a leaf spring and of abimetallic actuator which may be used in the switch body part of FIGS.1A to 1E;

FIG. 3 is an enlarged top plan view similar to FIG. 1A and showing inbroken lines the position of the leaf spring and bimetallic actuator;

FIG. 4 is a cross-sectional side elevation view similar to FIG. 1B butshowing the leaf spring and bimetallic actuator in place;

FIGS. 5A to 5I are views showing a second embodiment of the presentinvention with the leaf spring and bimetallic actuator omitted andfurther omitting a closure member, FIGS. 5A, 5B and 5C showing top plan,cross-sectional and bottom plan views respectively, FIGS. 5D and 5Eshowing perspective views, and FIGS. 5F to 5I showing other detail viewsas will be described more fully hereinafter;

FIG. 6 is a plan view showing the form of leaf spring used in the secondembodiment, the spring being shown in the form in which it is producedfrom continuous spring metal strip by a continuous stamping operationand before its final tooling for incorporation into the switch;

FIG. 7 is a plan view similar to that of FIG. 6 but showing the form ofbimetallic actuator that is used in the second embodiment;

FIGS. 8A to 8D are views showing the formation of the metal terminalparts of the second embodiment, FIG. 8A being a top plan view showinghow the metal parts for a plurality of switches are produced fromcontinuous metal strip by a continuous stamping operation, FIGS. 8B and8C being sectional views and FIG. 8D being a perspective view showingthe form of the metal parts as incorporated into a single mouldedplastics switch body and cropped from the continuous strip;

FIGS. 9A to 9D are views similar to those of FIGS. 8A to 8D and showingan alternative form of metal parts; and

FIGS. 10A to 10C are views similar to those of FIGS. 9A to 9C andshowing a modified form of the alternative metal parts.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The views in FIGS. 1 to 4 of the accompanying drawings of the firstembodiment show the switch to an enlarged scale and the dimensionsindicated are the actual dimensions of the switch in millimetres. Themoulded plastics body portion 1 of the switch is thus generallyrectangular with dimensions of 12.35 mm×8.0 mm×3.4 mm and the first andsecond terminal conductors 2,3 project outwardly by a further 4.5 mm. Atop cover for the switch is not shown in the drawings but has athickness of 0.5 mm. The switch thus has such small overall size that itmay conveniently be supplied in a bandolier suitable for use byautomatic component insertion equipment. Of course, these dimensions areexemplary only and differently sized switches could be constructed,particularly for use in different applications.

Within the body portion 1 of the switch there is defined a chamber 4which has dimensions of the order of 9.0 mm×7.0 mm×2.4 mm, and anupstand 5 occupies part of this chamber. The first terminal conductor 2,formed as a cruciform metal stamping as can best be seen in FIG. 1A, ismoulded into the body portion 1 at one end thereof with its forward end2', that is to say its end which extends furthest into the switch bodyportion 1, locating in a recess in the upper surface of the upstand 5,and its cross part 2" exposed within the chamber. The second terminalconductor 3 is moulded into the opposite end of the switch body portion1 at a lower level than the first conductor 2 and comprises a generallyT-shaped metal stamping the head 3' of which is exposed at the bottom ofthe chamber 4 defined within the body portion 1 of the switch.

FIG. 2 shows the shape of the leaf spring 6 that is incorporated intothe switch of FIGS. 1 to 4 and correspondingly shows the shape of thebimetallic actuator 7 of the switch, these two componentsadvantageously, in this embodiment, being of the same shape and beingformed with one and the same tooling in automatic manufacture of theswitch. As shown, the leaf spring and bimetallic actuator each comprisesa dished blade of appropriate spring or bimetallic material having agenerally U-shaped cut-out 8 which defines a tongue 9 between legs 10which are bridged by a bridging portion 11. The moving contact of theswitch is constituted by a silver contact 12 welded to the underside ofthe bridging portion of leaf spring 6 as can best be seen in FIG. 4. Thebimetallic actuator 7 locates on top of and in registry with the leafspring 6 and advantageously the two are secured together at theirtongues by means of a weak weld, as a sub-assembly, before being securedto the forward portion 2' of the terminal conductor 2 by virtue of theirtongues 9 being welded together thereto, formations 13 on the uppersurface of the upstand 5 aiding the location of the tongues 9 relativeto the body of the switch. The shape of the bimetallic blade 7 is suchas to enhance its responsiveness to through currents by increasing thecurrent density in the legs 10 and in the forward region of the bladewhere its bridging portion maintains physical and electrical contactwith the underlying leaf spring 6.

As with the switch described in WO 92/20086 aforementioned, the upstand5 provides furthermore for the support of the forward portion 2' ofconductor 2 which in turn provides support for tongue portion 9 of leafspring 6 and bimetallic actuator 7, whereas the legs 10 and bridgingportions 11 of these parts are free to move within the chamber 4. Byvirtue of this arrangement, the temperature responsive characteristicsof the switch can better be predetermined since switching operations areeffected substantially exclusively by flexure of the legs 10 about thestable position established for the tongues 9 by virtue of their supporton conductor 2. Furthermore, by supporting the tongues 9 in this way,the risk of stress cracking at the root of the tongue is reduced and theworking stresses in the bimetal and in the leaf spring are concentratedtowards their legs 10.

The fixed contact 14 of the switch is constituted in the embodiment inquestion by a silver contact welded to the head 3' of the T-shapedsecond conductor 3 where it is exposed within the internal chamber 4 ofthe switch. It could alternatively be formed by inlaying a silverportion into the conductor head 3', or by forming the conductor head 3'in accordance with the teachings of WO 92/14282, namely by forming theconductor 3 of copper or of a copper alloy having a thermal conductivityat least 90% that of copper, and more preferably 95% to 99% that of99.95% pure copper, and providing at least the conductor head 3' with athick plating layer of silver and antimony. By use of a thick plating(eg. 20 to 30 microns and preferably 40 microns thickness) comprisingfine silver (99.9% purity) with a small amount of antimony, typicallyabout 1% and particularly between 0.3% and 0.7%, on a conductor formedof copper or a high thermal conductivity copper alloy, the formation ofsilver powder during switching operations is inhibited and a switchingcontact life of the order of at least 70,000 switching cycles may beobtained.

In operation of the switch as thus described, the cold condition of theswitch is such that the bimetal 7 overcomes the spring bias of the leafspring 6 thereby holding the contact carried by the leaf spring incontact with the contact carried by the portion 3' of conductor terminal3. In this condition the leaf spring 6 shunts the bimetal 7 so that aproportion of the through current of the switch by-passes the bimetal.Whenever the temperature of the bimetal rises to a certain predeterminedlevel, as a result of thermal conduction from the switch environmentand/or as a result of heating of the bimetal by current flowtherethrough and/or through the adjoining leaf spring, the bimetal willmove to its oppositely dished configuration with a snap action therebyreleasing the leaf spring and allowing it to move under its own springbias into a contacts open position. When the bimetal subsequently coolssufficiently the switch will remake, the force developed by the bimetalovercoming the spring force of the leaf spring.

The use of the leaf spring 6 to shunt the bimetallic actuator 7increases the current capacity of the switch and increases the firstbreak time of the switch, that is to say the time that the switch takesto go open circuit for the first time after initiation of an excessivethrough-current. An extended first break time is advantageous for theavoidance of nuisance tripping in use of the switch in applications,such as motor protection applications for example, where the switch willinitially be subjected to a relatively high, but not abnormal, throughcurrent on start-up of the motor and the current will decrease as themotor picks up speed. The operation of the leaf spring in opposition tothe bimetal in this embodiment ensures that when operated the springremains in close physical and thermal contact with the bimetal. Thisresults in long off times for the switch, which enhances the protectivefunction of the switch.

In addition to the advantages abovementioned, the use of the leaf springto shunt the bimetal has the further advantage that a range of switchapplication specifications can be accommodated by use of but a singlebimetal material, the variations between different specifications beingaccommodated by selection of the physical and/or electricalcharacteristics of the spring material. Further variation can beachieved by selection of the bimetallic material also, and even furthervariation is obtainable by selection of the material of the terminalconductors 2 and/or 3, it being noted that the portions 2' and 2" of theconductor 2 are ideally located for use as a heat source to pump heatinto the bimetal if the conductor 3 is suitably formed for example of aresistive material and/or with thinned portions so that it develops heatin use of the switch.

Again, as with the switch described in WO 92/20086, the closure mayconveniently be moulded as an integral part of the switch body which ishingedly coupled thereto and may be ultrasonically welded shut afterassembly of the leaf spring and the bimetal into the switch chamber andspot welding of the tongues of the same to the forward part 2' ofconductor 2. The closure may be formed so as to isolate the chamber 4from the environment of the switch, or may alternatively be providedwith one or more openings.

The described switch is well suited to automatic manufacture andinstallation, comprises a minimum of parts and can be relativelyinexpensive, and is capable of miniaturisation for enhanced currentsensitivity. The switch as described is, however, but an example of whatis achievable within the ambit of the present invention andmodifications and variations are possible without departure from thespirit and scope of the invention. For example, whilst it isadvantageous to form the leaf spring and the bimetal in the same shape,this is not essential to the invention, and whilst it is advantageousthat the bimetal has no attachment to the leaf spring except where thetwo are welded together to the connector part 2', this also is notessential and the spring could have portions at its free (contactcarrying) end which engage loosely with the bimetal to ensure that anytendency of the spring-carried contact to stick to the fixed contact isresisted not only by the spring force of the leaf spring itself but alsois resisted by the force generated by the bimetal as it switches fromits cold to its hot condition. The illustrated arrangement of theterminal conductors 2,3 is also subject to modification, and plain wireconductors as disclosed in WO 92/20086 could alternatively be utilizedas could alternative surface mounting type terminals or alternativeterminal shapes, lengths and/or arrangements. The switch could also bemodified to incorporate series and/or parallel heating components asdiscussed in WO 92/20086.

Referring now to FIGS. 5 to 8 of the accompanying drawings, theseillustrate a second embodiment of the present invention whichincorporates some of the abovementioned possibilities for modificationand variation of the first embodiment. As with the drawings showing thefirst embodiment, the drawings showing the second embodiment are to anenlarged scale, 10 times actual size in the case of FIGS. 5A to 5F andFIG. 5I and 20 times actual in the case of FIGS. 5G and 5H, and thedimensions shown are in millimetres. Furthermore, the same referencenumerals are employed to designate parts of the second embodiment aswere used to designate like parts in the foregoing description of thefirst embodiment.

For the sake of conciseness, only the principal differences between thefirst and second embodiments will be described hereinafter and otherdetail changes will not be described, but will nonetheless be clear tothe appropriately skilled reader. Principally, it will be seen that themetal terminal parts of the second embodiment are designed to enable theswitch to be used selectively with either in-line or end-to-endterminations, that the leaf spring and the bimetallic actuator areloosely coupled together at their otherwise free ends, which enables aleaf spring to be used which develops a spring force tending for exampleto close the switch contacts rather than a contacts opening force as inthe first embodiment, and that the moulded plastics body portion of theswitch has an opening enabling a thinned section of a metal terminalpart in the switch to act as a heater without being totally enclosed inplastics material.

Referring more particularly to the drawings, FIGS. 8A to 8C show how theterminal parts 2 and 3 of the switch are formed by stamping from acontinuous metal strip 20 so as to form a series of terminal part setswhich remain attached to opposed longitudinal edges of the strip 20during subsequent automated manufacturing processes involving theattachment of the silver fixed contact 14 to a respective portion of theterminal 3, the moulding of the switch body 1, and the attachment of theleaf spring 6 and bimetallic actuator 7 to a respective portion of theterminal 2. As shown most clearly in FIG. 8A, the first terminal 2 has alaterally-inverted L-shape and is formed with a thinned portion 2-1which serves as an electrical resistance heater and an upstandingportion 2-2, see particularly FIG. 8B, which serves as a mounting forthe leaf spring 6 and the bimetallic actuator 7. The second terminal 3is generally h-shaped and has an upper limb 3-1 which, as shown in FIG.5A, is adapted to project from the same end of the switch body 1 as doesthe first terminal 2, and two spaced-apart lower limbs 3-2 and 3-3 whichare adapted to project from the opposite end of the switch body. As willbe appreciated by those possessed of the appropriate expertise, thisterminal arrangement enables the switch to be used with either in-lineor end-to-end terminations, the unwanted or redundant ones of secondterminal limbs 3-1, 3-2 and 3-3 being cropped off. As can be seen fromFIG. 5D, the terminal limbs 3-1, 3-2 and 3-3 are formed so as to haveportions which are exposed on the underside of the plastics bodymoulding 1 so that an anvil cropping tool can be placed under the switchand the unwanted terminal limbs cropped off flush with the side(s) ofthe switch body.

FIGS. 6 and 7 respectively show the form of the leaf spring 6 and of thebimetallic actuator 7, and it will be seen that in many respects theseparts are identical to the corresponding parts of the first embodimentas shown in FIG. 2. However, FIGS. 6 and 7 illustrate the preferredmanufacture of the leaf springs and bimetallic actuators from respectivemetal/bimetal strips in a continuous stamping process which provideselongate strips of series connected parts in a form which is welladapted for use in automatic switch manufacturing machinery.

FIG. 6 shows furthermore the formation of the spring blades 6 with anextended nose portion 11, as compared with the corresponding shape shownin FIG. 2, and a slot-like aperture 25 is formed in the elongated partof the nose portion 11 as shown. FIG. 7 shows the bimetal blade 7 formedwith a tab 26 extending from its nose portion 11. In assembly of theswitch, the nose portion 11 of the leaf spring 6 is bent upwardlythrough 90° so as to be upstanding from the general plane of the leafspring and so that the aperture 25 is located in the upwardly bent part,and the tab 26 of the bimetallic blade 7 is loosely received in theaperture 25 in the leaf spring 6. With the tongue portions 9 of the leafspring 6 and bimetallic blade 7 first lightly welded together as asub-assembly which is subsequently welded to the upstanding portion 2-2of the first terminal part 2, the engagement of the tab 26 of thebimetallic blade 7 in the aperture 25 of the leaf spring 25 couples thebimetal blade and leaf spring together in a loose fashion which requiresthem to move together but accommodates their individual movements sothat neither is unduly loaded by the other. With this arrangement, theleaf spring does not have to be formed so as to develop acontacts-opening spring force opposing the bimetal action and canalternatively be made neutral or compliant to the bimetal action or soas even to develop contacts-closing forces so as, as describedhereinbefore, to ensure that a positive contact pressure is obtained attemperatures below but closely approaching the switch openingtemperature. In the latter case, the leaf spring may be tooled so as tobe snap acting in its movement. Alternative means of coupling the leafspring and bimetallic actuator together in an accommodating fashion arepossible, but the described arrangement is simple and convenient and isparticularly well suited to automated manufacturing procedures in thatit necessitates only a minimum number of process steps.

Directing attention now to the thinned portion 2-1 of the first terminalpart 2 which is intended to form a heater in the switch, it will beseen, particularly from FIG. 5D, that the switch body 1 has an opening27 in its base at a location corresponding to the location in the switchof the heater part 2-1. This ensures that the heater operation is notunduly compromised by encasement of the heater in plastics material andensures efficient heat transfer to the bimetallic actuator by conductionand radiation.

Referring now to FIGS. 9A to 9D and FIGS. 10A to 10C, these showalternative terminal arrangements providing optional in-line orend-to-end utilizations and furthermore providing selectable shorter orlonger current paths through the switch. In the latter respect, it canbe seen that the terminal portion 2-2 to which the leaf spring andbimetal blade are to be welded is released from the terminal 2 at onelocation in the arrangement of FIGS. 9A to 9D and at a differentlocation providing different length current paths through the switch inthe arrangement of FIGS. 10A to 10C, the length of the current pathsdepending furthermore upon whether an in-line terminal configuration oran end-to-end terminal configuration is employed. By virtue of theseoptions, the resistance of the current path through the switch can beselected to be relatively low or relatively high and, in conjunctionwith selection of the material of the terminal elements 2 and 3 and/orof the leaf spring 6, the current sensitivity of the switch can beselected within a substantial range of possible sensitivities whichmeans, basically, that the same switch configuration can readily beadapted for different current loads and different switchingapplications.

As with the first embodiment, the second embodiment is susceptible tomodification and variation without departure from the spirit and scopeof the invention as set forth in the appended claims. The inventionenables the automatic manufacture of a series of basically similarswitches spanning a wide switching range, for example from 4 amps to 36amps. Further options would be possible by omission of the leaf springand manufacturing switches otherwise identical to those hereinbeforedescribed but with the bimetal itself carrying the moving contact of theswitch.

I claim:
 1. A thermally-responsive switch comprising a molded plasticbody portion capturing therein first and second spaced-apart terminalconductors, a leaf spring cantilevered from one of said conductors andextending toward the other of said conductors, said leaf spring having afree end carrying a contact which constitutes a moving contact of theswitch and is adapted to cooperate in switching operations with saidother of the two conductors, and a snap-acting bimetallic actuatorcantilevered from said one of said conductors and extending towards saidother of said conductors, said bimetallic actuator cooperating with saidleaf spring such that thermally induced movements of said bimetallicactuator transfer to the leaf spring to determine a condition of theswitch, the bimetallic actuator being electrically shunted by the leafspring which makes electrical contact with both a fixed end and saidfree end of the cantilevered bimetallic actuator.
 2. Thethermally-responsive switch as claimed in claim 1, wherein the leafspring and the bimetallic actuator are similarly shaped.
 3. Thethermally-responsive switch as claimed in claim 1, wherein thebimetallic actuator comprises a dished bimetallic blade having agenerally U-shaped cut-out defining a central tongue extending between apair of external legs which are bridged by a bridging portion adjacent atip of the tongue, and the tongue of the bimetallic blade is secured tosaid one of said terminal conductors.
 4. The thermally-responsive switchas claimed in claim 3, wherein the tongue of the bimetallic blade andthe corresponding part of the leaf spring are commonly secured to saidone of said conductors.
 5. The thermally-responsive switch as claimed inclaim 3, wherein the moving contact of the switch is carried by a partof the leaf spring that corresponds to the bridging portion of thebimetallic actuator.
 6. The thermally-responsive switch as claimed inclaim 3, wherein the molded plastics body portion of the switch definesa chamber which accommodates the bimetallic actuator and the leafspring, and an upstand is defined within said chamber and providessupport of the tongue of the bimetallic blade and for the correspondingpart of the leaf spring.
 7. The thermally-responsive switch as claimedin claim 1, wherein the leaf spring is adapted to develop a spring forcetending to open the switch contacts, and wherein the bimetallic actuatoroverlies the leaf spring and is adapted to develop, when in coldcondition, a force which overcomes the spring force of the leaf spring.8. The thermally-responsive switch as claimed in claim 7, wherein thefree ends of the leaf spring and the bimetallic actuator are devoid of amechanical coupling.
 9. The thermally-responsive switch as claimed inclaim 1, wherein the leaf spring is adapted to develop a neutral springforce tending neither to open nor to close the switch contacts, and amechanical coupling is provided between the free ends of the leaf springand the bimetallic actuator.
 10. The thermally-responsive switch asclaimed in claim 1, wherein the leaf spring is adapted to develop aneutral spring force tending to close the switch contacts, and amechanical coupling is provided between the free ends of the leaf springand the bimetallic actuator.
 11. The thermally-responsive switch asclaimed in claim 9, wherein the bimetallic actuator overlies the leafspring.
 12. The thermally-responsive switch as claimed in claim 9,wherein the mechanical coupling comprises an aperture in one of the leafspring and the bimetallic actuator, and a member on the other of theleaf spring and the bimetallic actuator which loosely engages saidaperture.
 13. The thermally-responsive switch as claimed in claim 1,wherein the leaf spring is adapted to be snap-acting in its operation.14. The thermally-responsive switch as claimed in claim 1, wherein themolded plastic body portion of the switch defines a chamber whichaccommodates the leaf spring and the bimetallic actuator, the first andsecond terminal conductors are molded into the body portion of theswitch at spaced locations so as to have exposed portions spaced apartfrom each other within said chamber and externally of the body portion,the bimetallic actuator and the leaf spring are directly welded to theexposed portion of said one of said conductors within the chamber, andthe contact carried by the leaf spring cooperates with a contact definedby or provided on the exposed portion of the other of the two conductorswithin the chamber.
 15. The thermally-responsive switch as claimed inclaim 14, wherein at least one of said first and second terminalconductors has plural exposed portions externally of the body portion ofthe switch.
 16. The thermally-responsive switch as claimed in claim 1,wherein the molded plastic body portion of the switch isolates theactive components of the switch from an external environment of saidswitch.
 17. The thermally-responsive switch as claimed in claim 1,wherein at least one of said terminal conductors comprises a resistanceheating portion adapted to pump heat into the bimetallic actuator. 18.The thermally-responsive switch as claimed in claim 1, wherein aresistance heating element is provided in parallel with the switchconductors.